The present invention relates to the field of data communication systems and to distributed two wire communication systems. More particularly the invention relates to a two wire communication system utilising a communication protocol in which an electrical bi-polar waveform is used for data transmission and power transfer over a two wire conductor system. The protocol is a physical layer protocol which defines the electrical waveform applied to a two wire conductor system acting as the transmission means. The two wire conductor system is used to interconnect a Controller and at least one module but preferably a multiplicity of modules. Throughout the specification the term Controller will be used to describe a device which controls the two wire conductor and the term Module will be used to describe a device which connects to the two wire conductor. In operation the communication protocol allows data communication between the Controller and the Modules across the two wire conductor by a digital communication methodology which is provided by the bi-polar waveform structure as defined in the protocol. Power transfer via the two wire conductor from the controller to the modules is also achieved by the bi-polar waveform structure as defined by the protocol.
Two wire conductor systems are known in the electrical engineering and electronics fields as a means of data and power transmission. Systems of this type generally known as Down Line Powered two wire conductor communication system (DLPCS) typically employ a controller which drives power into the two wire conductor systems, controls the timing of data transfer and transmits and receives data.
They also employ modules which draw power from the two wire conductor and transmit and receive data DLPCS""s can be classified into uni-polar DLPCS""s and bi-polar DLPCS""s. Most data and power transmission systems use different conductors for the transmission of data and the transmission of power. However, in low power applications where the length of the conductors between the Modules and the Controller is large, the transfer of power and data over the same two conductors can give large cost savings compared with multiple conductor systems. A unipolar DLPCS is a system where the Controller drives power down the two wire conductor using one voltage polarity. Data is transferred between the Controller and Modules by modulating the voltage (between voltages of the same polarity) and current (positive or negative). Power can be transferred from the Controller to the Modules while ever the line voltage are higher than the internal voltage of the Modules
A telephone system is an example of a uni polar DLPCS wherein power is drawn continuously from the line by the telephone which transmitts the voice signal by modulating the telephone""s impedance. The change of impedance modulates the voltage and current on the two wire communication line. Digital data transfer via the telephone DLPCS usually requires special modulation techniques using modems.
Another uni-polar DLPCS provides for a controller and multiple modules connected to a two-wire line and supports digital data only. The modules draw power whenever the voltage is high. Digital data is transmitted and received by the modules during periods when the voltage on the two -wire line is xe2x80x98lowxe2x80x99. The controller lengthens the xe2x80x98lowxe2x80x99 voltage period if a low impedance is detected across the two-wire line. Modules transmit data by modulating their impedance in the xe2x80x98lowxe2x80x99 voltage periods. An advantage of this signaling technique is that all modules connected to the two wire line receive all data transmitted over the two wire line. Module to module communication is also possible. The xe2x80x98lowxe2x80x99 voltage disables power transmission so the controller can easily detect the data signal. A disadvantage of this approach is that the xe2x80x98lowxe2x80x99 voltage has a low noise margin (the amount of noise voltage that is needed to corrupt the data). Another disadvantage is that it is difficult to adjust the speed of operation to accommodate different operational environments.
Another two wire utility data communication system for communicating utility meter readings over two wires is disclosed in Australian patent application 64664/94. The specification teaches communication systems used to transmit consumption data from a meter such as an electric, gas or water meter to remote meter reading units. The system described is very similar to a well known field known as M-BUS and provides for a controller and multiple modules connected to a two-wire line. The invention described comprises a reader/programmer for communicating with a utility meter or the like over at least two-wire lines. The system described includes means for generating an interrogate signal, means for demodulating a data signal received from the utility meter over the two-wire lines wherein the data signal consists of a current in the two-wire lines which varies in accordance with data being transmitted from the utility meter when the reader/programmer is connected via two wire lines and means for demodulating the data signal received from the utility meter over the wire lines wherein the data signal has characteristics which vary in accordance with the data being transmitted from the utility meter.
Each meter includes an encoder which converts consumption information displayed by a mechanical or electronic register associated with the meter to a form which can be transmitted over wires or the like to a remotely located meter reading unit. The specification also teaches the use of three wire communications for the system. When operating in the two wire mode the reader/programmer and encoder are inductively coupled.
The encoder includes circuitry for varying an impedance in accordance with data representing the quantity being measured by the meter to cause the current flowing between the encoder and the remote reader/programmer to be modulated in accordance with the data.
The system is relatively complicated in the coding and decoding of the signals in comparison to pulse duration signals and furthermore it does not permit use of 100% of data transmission time when transmitting power due to the nature of the uni-polar signaling used to transmit both power and data. The modules draw power while ever the voltage is xe2x80x98highxe2x80x99. The controller transmitts binary data to the modules by modulating the voltage between xe2x80x98highxe2x80x99 and zero voltage. The modules transmit data to the controller by modulating the line current on the two-wire line by varying the modules""s impedance. This system works well when the module""s data current is much larger than the power consumption current. It is therefore a disadvantage that power consumption is limited. Another disadvantage of this system is that modules can only receive data from the controller. Also the communication speed can be adjusted to meet unusual operating conditions but the speed is generally limited to fixed baud rates and it is difficult to change baud rates during operation.
Another known digital uni-polar DLPCS employs said two-wire protocol wherein the modules draw power when the voltage is high. In that system the controller transmits data to the modules by modulating the voltage between xe2x80x98highxe2x80x99 and zero voltage. The modules transmit data to the controller by modulating their impedance which modulates the line voltage and current. In this case all modules connected to the line can receive all data transmitted on the two-wire line as the modules modulate the voltage between xe2x80x98highxe2x80x99 and near zero voltage. The key disadvantage of this system is that very little power can be transmitted and communication speed has to be fixed.
Bi-polar DLPCS""s are systems where the controller drives power down the two wire conductor using an alternating voltage polarity.
Data is transmitted between the controller and modules by modulating the voltage and /or current. The module""s circuitry, which draws power from the line and enables data transfer by modulation of the voltage or the current, is more complex than the uni-polar interface circuitry of the DLPCS. Bi-polar systems have a number of advantages including the ability to detect a change of polarity more reliably and with less circuitry than a voltage level in the uni-polar systems. A further advantage is that the bi-polar system has twice the noise margin of a uni polar system for a given line voltage. Another advantage is that power transmission can occur during either polarity. The bi-polar signal supports binary communication, and low voltage periods, as in some uni polar systems, can be eliminated.
Australian patent 481585 (82170/75) discloses a signal transmission system for telemetering instruments and controllers in a process control system. It is an object of the invention described in that patent to provide a signal transmission system capable of transmitting data in terms of pulse duration together with power necessary to operate process instruments. The patent teaches that data transmission and power transmission are carried out alternately by changing the polarity of the line voltage. It is a further object of the invention disclosed in that patent to provide a signal transmission system capable of analogue-to-digital and digital-to-analogue conversion for signal transmission.
The system described is complex in that it employs diodes including a photocoupler which allow current to flow forward and which are capable of detecting the flow of current. As part of its operation, a signal start drives a converter so that process data is converted into a pulse width proportional to its analogue value. The patent teaches signal transmission and power transmission carried out alternately by changing the polarity of the loop current. Alternatively, the value of current, the polarity of voltage or the value of voltage may be changed to set up transmission systems for signal and power. Instead of proportional relationship between pulse width and analogue value, a logararithmic, exponential, square or like relationship may be used for the two signals. During data transmission, (lie controller modulates the data pulse duration in response to a change of impedance caused by the modules. This has the advantage that all modules can receive all data transmitted on the line.
The key disadvantage of this system is that no useful information is transmitted during the power period, wasting a significant percentage of the available transmission communication time. Furthermore, there is no means to enable the transmission of power during the data periods when the controller is the originator of the data. The system described is said to be readily compatible with digital systems as well as analogue systems.
The primary disadvantage of the system described in that patent is that during periods of power transmission no useful data transmission can occur.
Another known two wire system is described in Japanese publication No.5003457 but this arrangement also suffers from the disadvantage that data transmission time is compromised during power transmission. In fact a significant amount of time is devoted to power transmission which prevents data transmission during that period.
The prior art arrangements for transmitting data and power over the same two wires are disadvantaged by the fact that they devote a percentage of the total transmission time to the transfer of power during which phase no useful data is transmitted According to the prior art methodology, up to 50% of the available transmission time is used for transferring power to modules. Furthermore, all uni-polar prior art systems have much smaller noise margins and so are more susceptible to noise on long distance transmission.
Another prior art in digital bi-polar DLPCS involves power and data transmission by the controller to the modules at a xe2x80x98highxe2x80x99 bi-polar voltage and the modules respond using a low bi-polar voltage. In this system high communication speeds are possible. The xe2x80x98lowxe2x80x99 voltage transmission in this case can be made robust in the presence of noise because the modules directly drive the line voltages. The disadvantage of this approach is that the modules use significant power whenever they transmit data and the length of time that can expire between power transmissions from the controller necessitates large energy storage devices in all modules. Up until the present, the art has not included a system which modulates the pulse duration of both polarities with data and allows power to be drawn from either polarity depending on the data being transmitted. It is now possible using this system to power and communicate with hundreds possibly thousands of modules on a low cost cable.
Whilst the industry has been well served by standard communication techniques where power and data are run in separate cables there has been a long felt want for a system which enables communication and power transmission over long distances say greater than 5 kilometers and particularly in environments where power voltage and current must be kept at low levels for safety reasons but without compromise to data transmission time. Such a system can be used to advantage in explosive mining environments where long distances require increased power transmission but regulation requires a power not to exceed a predetermined maximum and where data transmission is desirable 100% of the time.
The present invention seeks to overcome theaforesaid prior art disadvantages by providing a two wire communication system in which data and power are transferred over the same two wires using a bi-polar signal such that useful data can be transmitted independant of and during power transmission without loss of data transmission time.
According to the invention 100% of the transmission time is used in data transmission without affecting the transfer of power to the modules connected to the two wire conductor. According to the prior art in comparison. up to 50% of transmission time may be lost in transfering power to the modules.
The arrangement of the present invention utilises a bi-polar signal which has a very large noise margin. As each period is followed by a period of the opposite polarity and only one value (analogue or digital) is conveyed during each period, there is no fixed timing with respect to data. Also, the line speed can be increased or decreased at an arbitrary baud rate, because timing information for data sampling is passed with each transition of the polarity. Further, the arrangement of the invention can support full duplex protocols on a single two wire conductor system. The protocol supports two data channel for simultaneous independant data transmission in both directions.
It is one object of the invention to provide a two wire communication protocol for a communication system which is capable of transmitting data in terms of pulse duration together with power to at least one but preferably a multiplicity of modules connected to the two-wire conductor wherein useful data may be sent 100% of the time and allow the designer to specifiy the power duty cycle required for the application.
In one broad form the present invention comprises:
a two wire communication system capable of transmitting data and transferring power by means of an electrical bi-polar waveform across a two wire conductor from a controller in communication with said conductor to at least one module in communication with said controller and/or data from said at least one module to said controller; the system including a remote electrical device in communication with said controller;
wherein useable data may be continuously transmitted across said two wire conductor independant of and during power transmission between said controller and said at least one module without loss of data transmission time and without affecting the transfer of power to said at least one module.
In another broad form the present invention comprises:
a two wire communication system capable of transmitting data and transferring power by means of an electrical bi-polar waveform across a two wire conductor from a controller in communication with said conductor to at least one module in communication with said controller and/or data from said at least one module to said controller; the two wire system including a remote electrical device in communication with said controller;
wherein useable data may be continuously transmitted across said two wire conductor independant of and during power transmission between said controller and said at least one module without loss of data transmission time and without affecting the transfer of power to said at least one module; wherein the system is capable of using up to 100% of available transmission time to transmit data without affecting transfer of power to said at least one module, and wherein data and power transfer are effected through the same transmission line.
According to a preferred embodiment, a plurality of modules are connected in parallel and are each capable of transmitting and receiving data across said conductor.
Preferably, when the controller is transmitting data said modules are capable of drawing power and when each said modules transmit data they do not draw power from said controller nor from other modules.
Each module is capable of transmitting data by putting a low impedance signal across said two wire conductor wherein; said low impedance signal from said module for data transmission does not distort pulse duration allowing said controller to transmit data on the two-wire transmission line to any value independant of said impedance signal from each module.
The low impedance signal may convey analogue or digital information by either modulating the current, voltage or duration of said low impedance signal.
In a further broad form the present invention comprises:
a two-wire communication system for transmitting power and data between a controller and at least one module; said system including a two wire transmission line including, two data channels which allow two independant bi-directional communication channels between the controller and said at least one module wherein power may be drawn by said at least one module whilst ever the controller is transmitting data on either data channel; wherein when said module is transmitting data back to the controller no power transfer is possible to other modules.
In another broad form the present invention comprises: a two wire communication system for transmitting power and data between a controller and at least one module, said system including a two wire transmission conductor including two data channels which allow two independant bi directional communication channels between the controller and said at least one module wherein power may be drawn by said at least one module whilst ever the controller is transmitting data on either data channel; wherein when said module is transmitting data back to the controller no power transfer is possible to other modules; wherein the system includes a P-side channel allowing bi-directional data flow and power transfer from said controller to said at least one module and an N-side channel allowing bi-directional data flow and power transfer between the controller and said at least one module; wherein, when data is transmitted on said P-side channel, said controller modulates at the P-side pulse width to effect data transfer such that the system is capable of utilising up to 100% of the transmission time to transmit data without affecting the transfer of power across said two wire transmission conductor.
According to a preferred embodiment there is provided means at the controller and modules to convey data to connected remote electrical devices and receive data from said connected remote electrical devices. According to the preferred embodiment there are a plurality of sparsely separated modules connected in parallel across said conductor along with one controller which is connected in parallel across said two-wire conductor.
Preferably a bi-polar signaling scheme is used which provides high noise immunity. The two polarities of the two wire conductor are designated P-side and N-side, wherein, the pulse width of the P-Side pulses and N-side pulses are modulated by the controller to convey all data to all modules connected to the two-wire conductor.
Preferably, all modules connected to the conductor can take power from the conductor when the controller is transmitting data. The relative duration of the controller data compared with the module data determines the power duty cycle for the conductor.
Preferably, the power available to the modules connected to the conductor can be traded off against communication speed.
Modules connected to the conductor can pass data to the controller by putting low impedance across the conductor. The controller can then modulate the duration of the pulse in response to the low impedance signal from the module.
The low impedance signal from the module does not distort the duration of the pulse directly, so the controller can still force data on the conductor to any desired value independantly of the modules low impedance signal.
Further, the module""s low impedance signal may convey analogue or digital information by either modulating the current, voltage or duration of the low impedance signal.
The present invention will now be described according to a preferred but non limiting embodiment and with reference to the accompanying illustrations.